Films for medical containers are generally formed by polyethylene or polypropylene which has high chemical stability to acids, alkalis, salts, etc. Also known as films for use in forming such medical containers having a plurality-of-chambers-are single-layer or two-layer films having a resin layer which is prepared from a molten mixture of alpha-polyolefin resins, such as polyethylene and polypropylene, which are different in compatibility.
For example, Unexamined Japanese Patent Publication No. 4671/1990 discloses a single-layer film of resin mixture of two components, i.e., a linear low-density polyethylene and polypropylene, or a single-layer film of resin mixture of three components, i.e., a linear low-density polyethylene, polypropylene and ethylene-propylene copolymer. A two-layer film is also disclosed which comprises an outer layer of linear low-density polyethylene, and an inner layer of resin mixture of linear low-density polyethylene and polypropylene. In the resin mixtures mentioned, polypropylene has the highest melting point and is used mainly to ensure easily peelable sealing properties.
However, the single-layer film of two-component resin mixture contains in the single-layer forming resin a large amount of polypropylene which is lower than polyethylene in transparency, flexibility and impact resistance, so that the container prepared from this film is low in usefulness in respect of transparency, flexibility and impact resistance.
Further, the problem of nonuniformity of the components is encountered with the single-layer film of three-component resin mixture when the mixture is made into the film. Stated more specifically, the ethylene-propylene copolymer which is amorphous or low in crystallinity and lowest in melting point is greater than the other two components and especially much greater than polypropylene in fluidity, with the result that the resin components of the film are likely to be present unevenly, It is therefore difficult for the film to effectively exhibit transparency, flexibility and impact resistance which are the outstanding characteristics of the ethylene-propylene copolymer resin. Moreover, it is difficult to uniformly disperse polypropylene in polyethylene and to form a film which has excellent easily peelable sealing properties, that is, which contains polypropylene as uniformly dispersed therein. The single-layer film wherein the component resins differ greatly in melting point encounter another problem when heat sealed. When the component resin of the highest melting point (polypropylene) starts to melt, the component resin of the lowest melting point (ethylene-propylene copolymer) has been excessively melted to flow in the form of a liquid, greatly reducing the thickness of the film to be heat sealed. Accordingly, the film is not heat sealable properly to give the desired strength to containers.
On the other hand, the two-layer film disclosed in the above-mentioned publication has slightly higher transparency and flexibility than the single-layer films since a linear low-density polyethylene (L-LDPE) is used for the outer layer, whereas this film is still unsatisfactory. Additionally, the film has a problem in respect of impact resistance when made into containers, especially when the container is cooled to 5.degree. C. or lower. For these reasons, the two-layer film is not satisfactory for medical use and is not suited to use.
Further because much consideration is not given to the heat resistance of these films, containers formed of these films and having a plurality of chambers are likely to give undesirable results. When the container is sterilized with high-pressure steam or hot water under a high-temperature condition (e.g., 121.degree. C. for 20 minutes), the resin present in the film (linear low-density polyethylene or ethylene-propylene copolymer) fails to withstand the sterilizing temperature to melt or foam, deforming or rupturing the container or permitting a leak through the sealed portion. It is further likely that the polyethylene will dissolve out in the form of fine particles. At the easily peelable seal portion providing a partition between the chambers and rendering the partition easily openable, the linear low-density polyethylene or ethylene-propylene copolymer similarly fails to withstand the sterilizing temperature to melt, consequently increasing the seal strength of the easily peelable seal portion to a level comparable to the strength of the hermetic seal portions (strong seal portions) other than the easily peelable seal portion, hence low usefulness. The sterilization at the temperature of 121.degree. C. also lowers the transparency and flexibility which are not negligible items of evaluation since these properties greatly influence the handling of the container at the site of medical treatment.